The present invention relates to improvements in apparatus for changing the moisture content of paper webs or the like, especially for reducing the moisture content of wet paper webs in paper making machines. More particularly, the invention relates to improvements in drying apparatus for paper webs or the like wherein the web directly contacts the peripheral surfaces of two cylindrical roll-shaped dryers and is trained around a hollow drum which is disposed between the dryers and has a perforated cylindrical wall, and wherein a portion of an endless screen consisting of wire mesh or the like is disposed between the cylindrical wall of the drum and the web.
German Offenlegungsschrift No. 2,315,792 discloses a web drying apparatus wherein the drum is a suction drum and the dryers have perforated cylindrical walls whose orifices discharge streams of a hot gaseous drying medium. The first dryer (as considered in the direction of movement of the web) cooperates with an auxiliary belt which maintains the web in contact with the endless screen and further serves as a means for transferring the web onto the drum. To this end, the first dryer is in direct contact with the drum. Such construction is not entirely satisfactory because the web must pass through the nip of the first dryer and the drum whereby the resulting pressure affects the quality of the web. The just described drying apparatus further comprises a blower which is located downstream of the second dryer and serves as a means for separating the treated web from the peripheral surface of the second dryer. The second dryer is also in direct contact with the drum, i.e., the paper web must pass through the nip of the first dryer and the drum as well as through the nip of the drum and the second dryer. Such mode of reducing the moisture content is not satisfactory in connection with the making and treatment of high-quality papers. Moveover, the apparatus is quite expensive because it employs three devices which discharge a compressed hot gaseous fluid, namely the two dryers and the aforementioned blower.
U.S. Pat. No. 3,418,723 to Burgess et al. discloses an apparatus which establishes turbulent conditions in the area where a hot gas contacts the paper web. The web is in direct contact with the suction drum and with two rolls which flank the drum and serve as a means for facilitating the threading of the leader of a paper web through the drying apparatus. In addition, the rolls are in sealing engagement with neighboring portions of a hot-air hood which surrounds the suction drum. The suction drum confines centrally located means for withdrawing moisture-laden air and stationary means for sealing off that portion of the drum which is not surrounded by the paper web. The turbulent gas which is drawn into the suction drum subjects the paper web to extremely high tensional stresses which often lead to breakage of the web. Furthermore, the threading of the leader of the web into and through the apparatus is complex and time-consuming, even if the apparatus is equipped with the aforementioned endless screen. Moreover, the paper web is likely to adhere to the peripheral surfaces of the rolls (rather than to advance with the screen); this results in undesirable elongation of the web with attendant reduction of quality and increased likelihood of web breakage.